Prismatic telescope, binocular, or the like.



PATENTED MAR. 14., 1905.

I. AITOHISON. PRISMATIG TELESCOPE, BINOCULAR, OR THE LIKE.

APPLIOATION FILED D110. 7, 1903.

2 sminrs-snnn'r 1.

N0. 784,611. PATENTED MAR. 14, 1905. J. AITOHISON.

PRISMATIG TELESCOPE, BINOGULAR, OR THE LIKE. APPLICATION FILED D120. 7,1903.

2 SHEETS-SHEET 2.

iiiiiiii'iii'l mud Patented March 14, 1905.

PATENT OFFICE.

JAMES AITCHISON, OF LONDON, ENGLAND.

PRISMATIC TELESCOPE, BINOCULAR, OR THE LIKE.

SPECIFICATION forming part of Letters Patent No. 784,611, dated March14, 1905.

Application filed December 7, 1903. Serial No. 184,182.

To all whom, it may concern.-

Be it known that I, JAMEs AI'roHIsoN, optician, a subject of the King ofGreat Britain, residing at 14 Newgate street, in the city of London,England, have invented certain new and useful Improvements in PrismaticTelescopes, Binoculars, or the Like, of which the following is aspecification.

My invention relates to prismatic telescopes, fieldglasses, andbinoculars and to any instrument in which erection of the im age issecured by totally-reflecting prisms, as in the Porro telescopes orotherwise. In such instruments as hitherto made the light from theconvergent object-glass usually passes through two totally-reflectingprisms of the usual form, a certain interval being in most cases allowedbetween the prisms, with the object of allowing the focal length of theobj ect-glass to be made considerably greater than the length of thetelescope-body. Glasses so constructed have an advantage over theGalilean or ordinary field-glass in that they can have a much greaterfield of View, since the eye-point is beyond the eyepiece. Hitherto,however, prismatic glasses have been markedly inferior to Galileanglasses in illumination, owing to the small relative aperture of theformer. In order to secure the maximum illumination from any telescope,it is-necessary that the diameter of the obj ectglass should bear thesame ratio to the eflective maximum diameter of the pupil of the eye asthe magnification required. For example, if we take the maximum apparentdiameter of the pupil as five millimeters and a magnification of twelvediameters is re uired the diameter of the object-glass shou d be sixtymillimeters. In prismatic binocular glasses of this magnification asordinarily made the diameter of the object-glass is not more thantwenty-five millimeters, or five twelfths of the above, giving anillumination of only twenty-five one hundred and fortyfourths, or about.174 ofthe full amount. With prisms of ordinary size and glass it isuseless to increase the diameter of the obj ectglass beyond this, as theextra light would be cut off by the prisms.

According to my invention I obtain greater diameter of object-glass, andhence of illumination, by the following devices.

Reference is hereby made to the accompanying drawings, in which similarletters indicate corresponding parts.

Figure 1 shows the optical arrangements of my prism glass without themountings and case. Fig. 2 is a diagrammatical sketch of a modification.Fig. 3 is a similar sketch of a further modification. Fig. 4 is asectional elevation of a prismatic binocular constructed according to myinvention, and Fig. 5 is a sectional plan of Fig. 4.

First. The prisms a I) are brought close together, as shown, whichgreatly increases the angle between the extreme rays which can pass intothe prism. These rays, however, will not be totally reflected andreceived by the eyepiece unless they meet the reflectingsurface at anangle greater than the critical angle of the glass of the prisms. Forordinary boro silicate crown glass, which is usually employed for prismsand which has a refractive index of 1.5093, we have O. 0 S111. criticalan le 1.5093 .675 from which the critical angle is forty-one degreesthirty minutes approximately. Since the optic axis meets the surface atan angle of forty-five degrees, it follows that the semiangle of thecone of light formed by the objectglass in the glass cannot be greaterthan 45 41 30 3 30, or 5 15 in air, in order to be totally reflected,which angle is insullicient for apertures in which the diameter of theobject-glass is greater than 2 X focal length tan. 5 15, or about .18 ofthe focal length. In order, therefore, to have a glass with a magnifyingpower of twelve diameters, and consequently with an object-glass sixtymillimeters diameter, the focal length .18 which would be inconvenientlygreat for a field-glass.

Second. In order to obtain total reflection of rays from a largeobjective, such as c, without lengthening the instrument, I employ glassfor the prisms having a high refractive index, and consequently alowcritical angle.

would have to be 333 millimeters,

((2) better correction of chromatic and spherical aberrations.

Third. To further increase the illumination, I cement together the twoprisms a b and the field-lens d of the eyepiece with Canada balsam orother suitable medium, thereby eliminating reflections between theprismfaces but for greater rigidity I sometimes prefer to cement them toa thin plate 6 of parallel glass, as shown in Fig. 1. In Figs. 2 and 4the two prisms are shown cemented to the glass plate 6, the field-lens dand magnifying-lens g of the eyepiece being mounted in a separate tube,as usual with ordinary prismatic telescopes. In Fig. 3 the two prismsare shown cemented face to face, the field-lens d and the achromaticlenses and g being detached therefrom.

Fourth. Since the increase of the aperture of the object-glass 0 withoutincreasing the length of the telescope involves diminution of the focallength of the obj ect-glass and a corresponding diminution in theequivalent focal length of the eyepiece if the same magnification is tobe maintained, I sometimes prefer to make the eyepiece of threeachromatized elementsfor example,the cemented field-lens d abovementioned and two other achromatic lenses f and g, Figs. 1 and 3and inorder stillfurther to decrease the curvature and consequent aberrationsof the eyepiecelenses I prefer to make one or more of these achromaticlenses f g a combination contain ing an anomalous glass combining highrefractive index with low dispersion, such as the densest Baryta crownglass, which enables an achromatic combination of relatively short focusand large aperture to be made with shallower curves and a flat field.

Fifth. To increase the aperture or as an aid to the above, I employ aconcave (or biconcave) lens h, which is fixed between the object-glassand the first face of the prism a or the glass plate 6, or the concavelens may be cemented to a glass block h, which is itself cemented to theplate 6. This enables the aperture to be increased when ordinary glassis employed or to be still further increased when used in conjunctionwith prisms of dense glass. It has the further advantage of increasingthe equivalent focal length of the object-glass while retaining acomparatively short body, since the principle employed is the same asthat of the telephoto-lens or Barlow telescope.

It has the disadvantage, however, of giving a smaller field than can beobtained by the dense glass prisms alone, owing to this increase of theequivalent focal length. The lens b may in some cases be biconcave, thefront concavity being for the before-mentioned purposes and the rearconcavity being fashioned to correct undue aberration or curvature.

In Figs. 4 and 5 I have shown a pair of prismatic binoculars adapted tomyimproved system. The body consists of the, two tubes M and the twoplates or bars I? 7c, all of which I prefer to form in one integralpiece by cast ing the same in aluminium or other light metal ormaterial. Upon the upper end of each tube I mount a prism-frame, whichconsists of a top plate Z, vertical strut or wall m, and lower plate a.The upper plate has an opening coincident with the axis of the'eyepiece,which projects through the opening, as shown in Fig. 4, and the lowerplate is formed with an eccentric tubular extension which fits thetubular opening of the body-tubes i and is coincident with the axis ofthe objectlass.

6 The prisms a and I), with the intermediate lass plate 6, arepreferably mounted in the following manner: The glass plate 0 is formedwith an angular edge, as shown in Figs. 1 and 2, which takes into agroove formed in the rim 0, which is tightened thereupon by the screwadjustment 0, Fig. 5. The rim 0 is integral with or attached to avertical plate p, which is fastened to lie snugly against the inner wallof the vertical strut m, to which it is secured in any convenient way,such as by a conical nut r and a screw-threaded pin or stud p, attachedto the plate 10, as shown in Fig. 5. The top plate Z is provided with acover or flange plate Z, which is detachable from the plate Z to allowof the inclosing case 8 being removed. The two prism frames or boxes aregeared together within the top plate I" of the body in any well-knownman ner, so that the two prism-boxes are simultaneously operated tobring the eccentric eyepieces nearer together or to separate them in thewell-known way.

With my improved system the focusing may be effected by extensibleeyepieces or object-glasses or i by means of telescopic prism-boxes orby separating the latter more or less from the body, as found desirable.

I am aware that prismatic glasses have been produced in which the prismshave been combined or cemented together, also inwhich relatively largeobject-glasses have been employed; but in the latter case the actualaperture made use of has been reduced by stops or diaphragms to normalamounts. I am not aware, however, that any successful or other attempthas yet been made to considerably increase the aperture by scientificIIO methods, especially by the important method of increasing therefractive indeX of the prisms (and) or by utilizing a concave lens asabove.

In the above description I have considered the application of thesemethods only to the ordinary pair of Porro prisms; but is is evidentthat the devices of increasing the refractive index or of employing aconcave lens are applicable to increase the aperture of any glass inwhich erection is secured by total reflection by unsilvered surfaces atangles of fortyfive degreesas, for example, in the Roof prisms sometimesemployed. The use of a divergent lens in combination with such prismsalso enables the aperture to be increased] I claim 1. In a device of theclass described, the combination With a plurality of suitable refractiveprisms, of a concave lens located in front of the prisms and a casinghaving a lar e object-glass and an eyepiece, substantially as described.

2. In a device of the class described, the combination of a concavelens, a plurality of suitable refractive prisms, said lens located infront of the prisms to increase the lightrays and reduce the angle ofthe cone, a cas ing having a large object-glass and an eyepiece, andmeans for adjusting the said casing.

3. In a device of the class described, the combination of a biconcavelens, a plurality of suitable refractive prisms, said lens located inthe front of the prisms to increase the lightrays and reduce the angleof cone, a casing having a large object-glass and an eyepiece, and meansfor adjusting the said casing.

4. In a device of the class described, the combination of a lens havinga concave face, a plurality of suitable refractive prisms arranged inclose proximity to each other, said lens located in the front of theprisms'to increase the light-rays and reduce the angle of the cone, acasing'having a large obj ectglass and an eyepiece, and means foradjusting the said casing.

In testimony whereof I have hereunto set my hand in presence of tWosubscribing witnesses.

JAMES AITCIIISON.

, Witnesses:

GEORGE O. DOWNING, WALTER J. SKERTEN.

